The present invention relates to chain tensioners, and more particularly, to an improved plunger retention device therefor.
Chain tensioners are now well known in the art, particularly those wherein a hydraulic plunger assembly is employed to provide a force directed against a moveable guide which bears against a timing chain of an internal combustion engine, to take up slack in the timing chain. Examples of such tensioners are shown in U.S. Pat. Nos. 4,826,470 and 5,304,099, both of which are assigned to the assignee of the present invention and are incorporated herein by reference.
Although the present invention may be used advantageously in a chain tensioner of the type in which only a spring is utilized to bias the plunger into engagement with the chain guide, the invention is especially suited for use with a hydraulic chain tensioner, and will be described in connection therewith.
In a typical hydraulic chain tensioner, an output member is biased in a protruding direction against the chain guide by the combined force of a spring and hydraulic pressure, thereby maintaining a predetermined tension in the chain. A check valve in the hydraulic circuit of the tensioner prevents retraction of the plunger under conditions wherein the force exerted on the chain guide by the timing chain exceeds the spring force.
As is well known to those skilled in the art, after assembly of the chain tensioner, it is necessary to provide some means to retain the plunger within the housing of the chain tensioner, in nearly its fully retracted condition. Unfortunately, in its fully retracted position, the plunger is subjected to the maximum force of the biasing spring, urging the plunger axially out of the plunger bore, toward its protruding position. Therefore, the means for retaining the plunger, after assembly of the tensioner, is subjected to nearly the maximum biasing force of the plunger spring.
One known method of retaining the plunger within the housing bore has been by means of a mechanism referred to generally as a "grenade pin", i.e., a pin which is put in place, passing through the tensioner housing, and holding the plunger in its retracted position until the tensioner is assembled to the engine, with the output portion of the plunger in engagement with the chain guide. At that point in the assembly process, the grenade pin is removed, and the spring biases the plunger into engagement with the chain guide.
The use of the grenade pin to retain the plunger of a typical chain tensioner has several disadvantages. First, there is the need to dispose of the pin in the engine assembly plant, after it is removed from the chain tensioner. There is also the related concern that the pin may end up somewhere in the engine where it doesn't belong. Secondly, there is always the concern about the possibility of the grenade pin being inadvertently removed while the chain tensioner is being handled, in which case the spring could propel the plunger out of the chain tensioner, which could damage the plunger, and at the very least, would require re-assembly of the tensioner by personnel not being equipped to do so. Finally, the grenade pin is adapted to retain the plunger in only one position, i.e., the fully retracted position of the plunger, whereas it is sometimes necessary to remove the chain tensioner from the engine when servicing the engine, and in those cases, it is desirable for the plunger to remain in the position it was in, so that when the tensioner is reinstalled on the engine, the same chain tension will again be applied as had been previously applied.